Rotors for electrical machines, specifically for synchronous electrical machines are well known in the art. Especially in the case where strong neodymium iron boron magnets are used, the fastening of such magnets on the rotor becomes quite challenging. Different solutions for providing a fastening of such strong magnets have been proposed in the state of the art.
In a thesis submitted by Eckard Nipp (Permanent Magnet Motor Drives with Switched Stator Windings; Thesis submitted to the School of Electrical Engineering and Information Technology, KTH; Royal Institute of Technology of Electric Power Engineering Machines and Drives, Stockholm, 1999), the embedding of magnets into the rotor structure was proposed.
EP 2 187 503 (Traktionssysteme Austria GmbH) relates to a rotor for an electrical machine, wherein a multitude of openings are provided along the circumference of the rotor, permanent magnets being arranged within said openings. To reduce the magnetic leakage flux and enhance the stability of the rotor, the openings comprise voids arranged towards the neighbouring openings.
JP 2011/066373 (Nissan Motor Co. Ltd.) proposes to arrange two embedded magnets in a V-shape that opens towards the outer periphery of the rotor. A further permanent magnet is positioned parallel to the circumferential direction to the open part of the V-shape.
However, such embedding of magnets has the disadvantage of creating a magnetic short circuit and hence provides a decreased magnetic flux density in the air gap between the rotor and the stator.
EP 0 996 212 (Technische Universität Eindhoven) proposes to provide a rotor support upon which at least two poles are provided on the periphery, each pole comprising a permanent magnet. A screen made of highly conducting material is arranged on the surface of the permanent magnets. The screen is configured to face the stator of an electric machine.
EP 2 068 425 (E & A Forschungsinstitut Elmas) is also directed to the use of a bandage enrobing a rotor with magnets provided on an outer periphery of the rotor.
Use of retaining elements like bandages provides a good mechanical attachment of magnets to a rotor. However, eddy current loses are caused close to the air gap of the electrical machine, since alternating magnetic fields induced by the windings of a stator have a significant impact on magnets arranged close to said gap. This results in a temperature rise and a decrease of the efficiency of the electrical machine. Cooling of the magnets to reduce such heat losses is difficult since the retaining elements cause an additional heat insulation of the magnets. Further, retaining elements may be damaged during assembly or operation of the rotor and become detached of the magnets. Detachment of a retaining element during operation of the rotor may cause heavy damage to the rotor or the electrical machine.
Another approach is used by e.g. EP 1 860 755, where a circumferential array of magnet carriers is affixed to an outer rim of a rotor. An inverted U-shaped pole piece retainer made of non-magnetic material is affixed to each magnet carrier and is formed with an axially extending channel. A pole piece made of magnetic material is located adjacent to the radial outer surface of each magnet carrier and in the channel formed in its associated pole piece retainer.
WO 2009/068736 (Neorem Magnets OY) discloses a surface-mountable permanent magnet element to be mounted onto the periphery of a rotor. The permanent magnet element comprises at least one permanent magnet as well as a protective case enclosing said permanent magnet. The protective case includes a securing member for securing the element on the periphery of a rotor. Further, a housing formed of metal thin sheet covers the permanent magnet.
Such metallic caps arranged on top of permanent magnets with the goal of rigidly securing the magnets on a rotor have the disadvantage that significant eddy currents are generated in those metallic caps, which cause a decrease in the efficiency of the electrical machine.
A further approach is described in EP 1 777 795 (The General Electric Company), where a pole assembly including a permanent magnet block, a plurality of laminations comprising a pole cap mechanically coupled to the pole, as well as a plurality of laminations comprising a base plate mechanically coupled to said pole is arranged on the periphery of a rotor.
US 2013/0140932 (General Electric Company) proposes a rotor core for an electric machine, said rotor core comprising spokes arranged radially about the rotor core and being attached to the rotor core by means of a form-fit interlock with tenons radiating from said rotor core. Conductors are arranged within conductor openings located between two adjacent spokes.
However, these two arrangements both use a great number of separate parts and are thus cumbersome and expensive in manufacture.
US 2013/0169081 (Siemens Aktiengesellschaft) describes a rotor for an electrical machine comprising a plurality of segments. A fan is arranged at least between two of these segments. A plurality of segments is joined to form a rotor core, while end sheets are attached to the axial ends of the rotor core.